It will automatically get smaller because of perspective, but you can do it manually, too (however, this would be unrealistic). Uhrwerk already told you how to achieve it, check the manual for c_trace(), vec_rotate(), hit.x/ "target" and effect() or ent_decal().
"Falls das Resultat nicht einfach nur dermassen gut aussieht, sollten Sie nochmal von vorn anfangen..." - Manual
